Spermatogenesis in Drosophila is marked by a period of intense transcriptional activity and cell growth. This growth phase, which coincides with meiotic prophase, provides all or nearly all the transcripts required for meiosis and spermiogenesis. Although a number of genes are known to be specifically expressed during this period, little is known about the trans-acting factors required for regulation of spermatocyte gene expression. This proposal focuses on a binding activity in testis extracts that is capable of specific binding to a 5' regulatory element that directs spermatocyte-specific transcription. The first specific aim will use in vitro binding and transcription assays to define the binding activity and its target. The second aim will use several alternative approaches one-hybrid screening, biochemical purification, and mutant identification to clone the gene encoding the binding factor. The third aim will examine the in vivo function of the cloned factor through both inactivation of the gene and ectopic expression.